1. Technical Field
The present invention relates generally to a priority and pre-emption service system for satellite related communication, and more particularly, to a priority and pre-emption system for a mobile satellite communication system utilizing a central controller or network operations controller for coordination of same.
2. Background Art
An overview of the satellite network system is illustrated in FIG. 1. The satellite network system design provides the capability for METs and FESs to access one or more multiple beam satellites located in geostationary orbit to obtain communications services.
The heart of the satellite network system for each of the networks is the Network Control System (NCS) which monitors and controls each of the networks. The principal function of the NCS is to manage the overall satellite network system, to manage access to the satellite network system, to assign satellite circuits to meet the requirements of mobile customers and to provide network management and network administrative and call accounting functions.
The satellites each transmit and receive signals to and from METs at L-band frequencies and to and from Network Communications Controllers (NCCs) and Feederlink Earth Stations (FESs) at Ku-band frequencies. Communications at L-band frequencies is via a number of satellite beams which together cover the service area. The satellite beams are sufficiently strong to permit voice and data communications using inexpensive mobile terminals and will provide for frequency reuse of the L-band spectrum through inter-beam isolation. A single beam generally covers the service area.
The satellite network system provides the capability for mobile earth terminals to access one or more multiple beam satellites located in geostationary orbit for the purposes of providing mobile communications services. The satellite network system is desired to provide the following general categories of service:
Mobile Telephone Service (MTS). This service provides point-to-point circuit. switched voice connections between mobile and public switched telephone network (PSTN) subscriber stations. It is possible for calls to be originated by either the mobile terminal or terrestrial user. Mobile terminal-to-mobile terminal calls are also supported.
Mobile Radio Service (MRS). This service provides point-to-point circuit switched connections between mobile terminal subscriber stations and subscriber stations in a private network (PN) which is not a part of the PSTN. It is possible for calls to be originated from either end. Mobile terminal-to-mobile terminal calls are also supported.
Mobile Telephone Cellular Roaming Service (MTCRS). This service provides Mobile Telephone Service to mobile subscribers who are also equipped with cellular radio telephones. When the mobile terminal is within range of the cellular system, calls are serviced by the cellular system. When the mobile terminal is not in range of the cellular system, the MTCRS is selected to handle the call and appears to the user to be a part of the cellular system. When the mobile terminal is not in range of the cellular system, the MTCRS is selected to handle the call and appears to the user to be a part of the cellular system. It is possible for calls to be originated either from the MET or the PSTN. Mobile terminal-to-mobile terminal calls are also supported.
Mobile Data Service (MDS). This service provides a packet switched connection between a data terminal equipment (DTE) device at a mobile terminal and a data communications equipment (DCE)/DTE device connected to a public switched packet network. Integrated voice/data operation is also supported.
The satellites are designed to transmit signals at L-band frequencies in the frequency band 1530-1559 MHz. They will receive L-band frequencies in the frequency band 1631.5-1660.5 MHz. Polarization is right hand circular in both bands. The satellites will also transmit in the Ku frequency band, 10,750 MHz to 10,950 MHz, and receive Ku-band signals in the frequency band 13,000 to 13,250 MHz.
The satellite transponders are designed to translate communications signals accessing the satellite at Ku-band frequencies to an L-band frequency in a given beam and vice versa. The translation will be such that there is a one-to-one relation between frequency spectrum at Ku-band and frequency spectrum in any beam at L-band. The satellite transponders will be capable of supporting L-band communications in any portion of the 29 MHz allocation in any beam.
Transponder capacity is also provided for Ku-band uplink to Ku-band down-link for signalling and network management purposes between FESs and NCCs. The aggregate effective isotropic radiated power (AEIRP) is defined as that satellite e.i.r.p. that would result if the total available communications power of the communications subsystem was applied to the beam that covers that part of the service area. Some of the key performance parameters of the satellite are listed in FIG. 2.
The satellite network system interfaces to a number of entities which are required to access it for various purposes. FIG. 3 is a context diagram of the satellite network system illustrating these entities and their respective interfaces. Three major classes of entities are defined as user of communications services, external organizations requiring coordination, and network management system.
The users of satellite network communications services are MET users who access the satellite network system either via terrestrial networks (PSTN, PSDN, or Private Networks) or via METs for the purpose of using the services provided by the system. FES Owner/Operators are those organizations which own and control FESs that provide a terrestrial interface to the satellite network. When an FES becomes a part of the satellite network, it must meet specified technical performance criteria and interact with and accept real-time control from the NCCs. FES Owner/Operators determine the customized services that are offered and are ultimately responsible for the operation and maintenance of the FES. Customers and service providers interact with the Customer Management Information System within the Network Management System.
The satellite network system interfaces to, and performs transactions with, the external organizations described below:
Satellite Operations Center (SOC): The SOC is not included in the satellite network ground segment design. However, the satellite network system interfaces with the SOC in order to maintain cognizance of the availability of satellite resources (e.g. in the event of satellite health problems, eclipse operations, etc.) and, from time to time, to arrange for any necessary satellite reconfiguration to meet changes in traffic requirements.
NOC: The satellite network system interfaces with the satellites located therein via the NOC for a variety of operational reasons including message delivery and coordination.
Independent NOCs: The satellite network system interfaces with outside organizations which lease resources on satellite network satellites and which are responsible for managing and allocating these resources in a manner suited to their own needs.
Other System NOCs: This external entity represents outside organizations which do not lease resources on satellite network satellites but with whom operational coordination is required.
The satellite network management system (NMS) is normally located at an administration""s headquarters and may comprise three major functional entities; Customer Management Information System (CMIS), Network Engineering, and System Engineering (NE/SE). These entities perform functions necessary for the management and maintenance of the satellite network system which are closely tied to the way the administration intends to do business. The basic functions which are performed by CMIS, Network Engineering, and System Engineering are as follows:
Customer Management Information System: This entity provides customers and service providers with assistance and information including problem resolution, service changes, and billing/usage data. Customers include individual MET owners and fleet managers of larger corporate customers. Service providers are the retailers and maintenance organizations which interact face to face with individual and corporate customers.
Network Engineering: This entity develops plans and performs analysis in support of the system. Network Engineering analyzes the requirements of the network. It reconciles expected traffic loads with the capability and availability of space and ground resources to produce frequency plans for the different beams within the system. In addition, Network Engineering defines contingency plans for failure situations.
System Engineering: This entity engineers the subsystems, equipment and software which is needed to expand capacity to meet increases in traffic demands and to provide new features and services which become marketable to subscribers.
The satellite network system comprises a number of system elements and their interconnecting communications links as illustrated in FIG. 4. The system elements are the NOC, the NCC, the FES, the MET, the Remote Monitor Station (RMS), and the System Test Station (STS). The interconnecting communications links are the satellite network Internetwork, terrestrial links, the MET signaling channels, the Interstation signaling channels, and the MET-FES communications channels. The major functions of each of the system elements are as follows:
NOC. The NOC manages and controls the resources of the satellite network system and carries out the administrative functions associated with the management of the total satellite network system. The NOC communicates with the various internal and external entities via a local area network (LAN)/wide area network (WAN) based satellite network Internetwork and dial-up lines.
NCC. The NCC manages the real time allocation of circuits between METs and FESs for the purposes of supporting communications. The available circuits are held in circuit pools managed by Group Controllers (GCs) within the NCC. The NCC communicates with the NOC via the satellite network Internetwork, with FESs via Ku-to-Ku band interstation signaling channels or terrestrial links, and with mobile terminals via Ku-to-L band signaling channels.
FES. The FES supports communications links between METs, the PSTN, private networks, and other MTs. Once a channel is established with an MET, call completion and service feature management is accomplished via In-Band signaling over the communication channel. Two types of FESs have been defined for the satellite network system; Gateway FESs and Base FESs. Gateway FESs provide MTS, MRS, MTCRS and NR services. Base FESs are for like services and/or value added services.
MET. The MET provides the mobile user access to the communications channels, and services provided by the satellite network system. A range of terminal types has been defined for the satellite network system.
RMS. The RMS monitors L-band RF spectrum and transmission performance in specific L-band beams. An RMS is nominally located in each L-band beam. Each RMS interfaces with the NOC via either a satellite or terrestrial link.
STS. The STS provides an L-band network access capability to support FES commissioning tests and network service diagnostic tests. The STS is collocated with, and interfaced to, the NOC.
Communications channels transport voice, data and facsimile transmissions between METs and FESs via the satellite. Connectivity for MET-to-MET calls is accomplished by double hopping the communications channels via equipped FESs. Signaling channels are used to set up and tear down communications circuits, to monitor and control FES and MET operation, and to transport other necessary information between network elements for the operation of satellite network. The system provides Out-of-Band and Interstation signaling channels for establishing calls and transferring information. In-Band signaling is provided on established communications channels for supervisory and feature activation purposes. A detailed description of the satellite network signaling system architecture is provided in L. White, et al., xe2x80x9cNorth American Mobile Satellite System Signaling Architecture,xe2x80x9d AIAA 14th International Communications Satellite Conference, Washington, DC (March 1992), incorporated herein by reference.
The satellite network Internetwork provides interconnection among the major satellite network ground system elements such as the NOCs, NCCs, and Data Hubs, as well as external entities. Various leased and dial-up lines are used for specific applications within the satellite network system such as backup interstation links between the NCC and FESs and interconnection of RMSs with the NOC.
The primary function of the NOC is to manage and control the resources of the satellite network system. FIG. 5 is a basic block diagram of the NOC and its interface. The NOC computer is shown with network connections, peripheral disks, fault tolerant features, and expansion capabilities to accommodate future growth. The NOC software is represented as two major layers, a functional layer and a support layer. The functional layer represents the application specific portion of the NOC software. The support layer represents software subsystems which provide a general class of services and are used by the subsystems in the functional layer.
The application specific functions performed by the NOC are organized according to five categories: fault management, accounting management, configuration management, performance management, and security management. The general NCC Terminal Equipment (NCCTE) configuration showing constituent equipment includes: processing equipment, communications equipment, mass storage equipment, man-machine interface equipment, and optional secure MET Access Security Key (ASK) storage equipment. The Processing Equipment consists of one or more digital processors that provide overall NCC control, NCS call processing, network access processing and internetwork communications processing.
The Communications Equipment consists of satellite signaling and communications channel units and FES terrestrial communication link interface units. The Mass Storage Equipment provides NCC network configuration database storage, call record spool buffering an executable program storage. The Man-Machine Interface Equipment provides operator command, display and hard copy facilities, and operator access to the computer operating systems. The MET ASK storage Equipment provides a physically secure facility for protecting and distributing MET Access Security Keys.
The NCCTE comprises three functional subsystems: NCCTE Common Equipment Subsystem, Group Controller Subsystem, and Network Access Subsystem. The NCCTE Common Equipment subsystem comprises an NCC Controller, NCCTE mass storage facilities, and the NCCTE man-machine interface. The NCC Controller consists of processing and database resources which perform functions which are common to multiple Group Controllers. These functions include satellite network Internetwork communications, central control and monitoring of the NCCTE and NCCRE, storage of the network configuration, buffering of FES and Group Controller call accounting data, transfer of transaction information to the Off-line NCC and control and monitoring of FESs.
The Mass Storage element provides NCC network configuration database storage, call accounting data spool buffering, and NCCTE executable program storage. The Man-machine Interface provides Operator command and display facilities for control and monitoring of NCC operation and includes hard copy facilities for logging events and alarms. A Group Controller (GC) is the physical NCC entity consisting of hardware and software processing resources that provides real time control according to the CG database received from the NOC.
The Group Controller Subsystem may incorporate one to four Group Controllers. Each Group Controller maintains state machines for every call in progress within the Control Group. It allocates and de-allocates circuits for FES-MET calls within each beam of the system, manages virtual network call processing, MET authentication, and provides certain elements of call accounting. When required, it provides satellite bandwidth resources to the NOC for AMS(R)S resource provisioning. The Group Controller monitors the performance of call processing and satellite circuit pool utilization. It also performs MET management, commissioning and periodic performance verification testing.
The Network Access Subsystem consists of satellite interface channel equipment for Out-of-Band signaling and Interstation Signaling which are used to respond to MET and FES requests for communications services. The Network Access Processor also includes MET communications interfaces that are used to perform MET commission testing. In addition, the subsystem includes terrestrial data link equipment for selected FES Interstation Signaling.
The principal function of the FES is to provide the required circuit switched connections between the satellite radio channels, which provide communications links to the mobile earth terminals, and either the PSTN or PN. FESs will be configured as Gateway Stations (GS) to provide MTS and MTCRS services or Base Stations to provide MRS services (described in detail below). Gateway and Base functions can be combined in a single station.
The FES operates under the real time control of the Network Communications Controller (NCC) to implement the call set-up and take-down procedures of the communications channels to and from the METs. Control of the FES by the NCC. is provided via the interstation signaling channels. An FES will support multiple Control Groups and Virtual Networks. The FES is partitioned into two major functional blocks, the FES RF Equipment (FES-RE) and the FES Terminal Equipment (FES-TE). The principal function of the FES-RE is to provide the radio transmission functions for the FES. In the transmit direction it combines all signals from the communications and interstation signaling channel unit outputs from the FES-TE, and amplifies them and up-convert these to Ku-Band for transmission to the satellite via the antenna. In the receive direction, signals received from the satellite are down-converted from Ku-Band, amplified and distributed to the channel units within the FES-TE. Additional functions include satellite induced Doppler correction, satellite tracking and uplink power control to combat rain fades.
The principal function of the FES-TE is to perform the basic call processing functions for the FES and to connect the METs to the appropriate PSTN or PN port. Under control of the NCC, the FES assigns communications channel units to handle calls initiated by MET or PSTN subscribers. The FES-TE also performs alarm reporting, call detail record recording, and provision of operator interfaces.
For operational convenience, an FES may in some cases be collocated with the NCC. In this event, the NCC RF Equipment will be shared by the two system elements and the interstation signaling may be via a LAN. Connection to and from the PSTN is via standard North American interconnect types as negotiated with the organization providing PSTN interconnection. This will typically be a primary rate digital interconnect. Connection to and from private networks is via standard North American interconnect types as negotiated with the organization requesting satellite network service. This will typically be a primary rate digital interconnect for larger FESs or an analog interconnect for FESs equipped with only a limited number of channels may be employed.
It has been discovered that there is a general need for an integrated mobile telephone that can be used to transmit to, and receive from, to communicate in a virtual network arrangement that allows each member of the group to hear what any other user is saying. Each member of the group can also talk when needed. The system behaves like a radio multi-party line where several parties communicate over the same communication channel. Public services and law enforcement agencies are typical users of this service, which is normally provided by either traditional terrestrial radio networks or by the more recent trunked radio systems. These trunked systems, generally in the 800-900 MHz band, provide groups of end users with virtual network systems by assigning frequencies on a demand basis. In this connection, however, it has been discovered that an integrated mobile communication device is needed that provides this ability to communicate in a virtual network of a satellite communications system.
It has also been discovered the need for a nationwide and regional point-to-multipoint mobile communication service that is not limited in coverage.
It is a feature and advantage of the present invention to provide an integrated mobile telephone that can be used to transmit and receive in a virtual network arrangement that allows each member of the group to hear what any other user is saying.
It is another feature and advantage of the present invention to permit each member of the group to talk when needed, and to provide a system that behaves like a radio multi-party line.
It is a further feature and advantage of the present invention to provide an integrated mobile communication device that can communicate in a virtual network of a satellite network.
It is another feature and advantage of the present invention to provide an inexpensive virtual network satellite service to the owner of the group.
It is another feature and advantage of the present invention to minimize the call set-up time for one shared circuit per virtual network.
It is another feature and advantage of the present invention to generally effectively and efficiently effectuate transmissions between mobile communication devices and the satellite network in a virtual network environment by utilizing an efficient communication protocol.
It is another feature and advantage of the invention to provide a nationwide and regional point-to-multipoint mobile communication service that is not limited in coverage.
The present invention is based, in part, on the desirability of providing point-to-multipoint circuit switched connections between mobile terminal subscriber stations and a central base station in a virtual network. Mobile users are able to listen to two-way conversations and to transmit.
To achieve these and other features and advantages of the present invention, a mobile communication system is provided in a mobile satellite system. The mobile satellite system includes a satellite communication switching office having a satellite antenna for receiving/transmitting a satellite message via a satellite from/to a vehicle using a mobile communication system, a satellite interface system, a central controller receiving/transmitting the satellite message from/to the satellite communication switching office issued from the vehicle via the satellite and the satellite interface system. The mobile communication system includes a user interface system providing a user interface through which a user has access to services supported by the mobile satellite system, and an antenna system providing an interface between the mobile communication system and the mobile satellite system via is the satellite interface system, and receiving a first satellite message from the satellite and transmitting a second satellite message to the satellite. The antenna system includes an antenna including one of a directional and an omnidirectional configuration, a diplexer, an amplifier, a low noise amplifier, a beam steering unit when the antenna is of the directional configuration, and at least one of a compass and sensor to determine vehicle orientation. The mobile communication system also includes a transceiver system, operatively connected to the antenna system, including a receiver and a transmitter. The transmitter converts the second satellite message including at least one of voice, data, fax and signaling signals into a modulated signal, and transmits the modulated signal to the antenna system. The transmitter includes an amplifier, a first converter and associated first frequency synthesizer, a modulator, an encoder, multiplexer, scrambler and frame formatter for at least one of voice, fax, and data. The receiver accepts the first satellite message from the antenna system and converts the first satellite message into at least one of voice, data, fax and signaling signals, at least one of the voice, data and fax signals routed to the user interface system. The receiver includes a second converter with an associated second frequency synthesizer, a demodulator, a decoder, demultiplexer, descrambler and frame unformatter for at least one of voice, fax, and data. The mobile communication system also includes a logic and signaling system, operatively connected to the transceiver, controlling initialization of the mobile communication system, obtaining an assigned outbound signaling channel from which updated system information and commands and messages are received. The logic and signaling system configures the transceiver for reception and transmission of at least one of voice, data, fax and signaling messages, and controls protocols between the mobile communication system and the mobile satellite system, and validating a received signalling messages and generating codes for a signaling message to be transmitted.
In one embodiment of the invention, a system for providing satellite communication between multiple users in a virtual network arrangement includes first and second mobile earth terminals (METs) responsively connected to and registering with the mobile satellite system. The first MET selects a virtual network identifier (VN ID) representing a virtual network group including the first and second METs to establish voice communication therewith and transmits the VN ID to a central controller. The central controller receives the VN ID from the first MET, allocates a frequency for the virtual network group, and broadcasts the message to the virtual network group including the second MET informing the virtual network group of the allocated frequency and the voice communication associated therewith. The second MET tunes to the frequency in response to the message broadcast by the central controller.
In another embodiment of the invention, a method of providing satellite communication between multiple users in a virtual network arrangement includes the steps of first and second mobile earth terminals (METs) registering with the mobile satellite system, the first MET selecting a virtual network identifier (VN ID) representing a virtual network group including the first and second METs to establish voice communication therewith. The method also includes the steps of the first MET transmitting the VN ID to the central controller, the central controller receiving the VN ID, allocating a frequency for the virtual network group, and broadcasting the message to the virtual network group including the second MET informing the virtual network group of the allocated frequency and the voice communication associated therewith. The method also includes the steps of the second MET tuning to the frequency in response to the message broadcast by the central controller.
In another. embodiment of the invention, the method also includes the steps of a third MET included in the virtual network group registering with the mobile satellite system, and the central controller broadcasting the message to the virtual network group including the third MET informing the virtual network group of the allocated frequency and the voice communication associated therewith. The method also includes the steps of the third MET tuning to the frequency in response to the message broadcast by the central controller.
According to the invention, the central controller advantageously controls the virtual network satellite communication including virtual network parameters used by the first and second METs.
The central controller advantageously collects billing information regarding the virtual network satellite communication and transmits the billing information to the mobile satellite system. The mobile satellite system optionally charges a service fee to a customer that has requested the virtual network arrangement instead of each of the individual users in the virtual network group thereby consolidating the billing transactions and permitting a single customer to monitor communication charges.
In another embodiment of the invention, the method includes the steps of the first MET selecting the virtual network identifier (VN ID) representing a virtual network group including the first MET and a non-MET serviced by one of a public switched telephone network and a cellular network to establish voice communication therewith, and the first MET transmitting the VN ID to the central controller. Additionally, the method includes the central controller receiving the VN ID, determining that the virtual network group includes the non-MET, and broadcasting a non-MET message to either the public switched telephone network or the cellular network including the voice communication associated therewith, and either the public switched telephone network or the cellular network receiving the non-MET message from the central controller and transmitting the non-MET message to the non-MET to establish the virtual network arrangement between the MET and the non-MET.
In another embodiment of the invention, the NOC manages and controls the resources of the satellite network system and carries out the administrative functions associated with the management of the total satellite network system. The NOC communicates with the various internal and external entities via a local area network (LAN)/wide area network (WAN) based satellite network Internetwork and dial-up lines.
The NOC""s network management functions include measuring the usage of resources by customers to enable predictions of what changes to make in the future deployment of resources. Such resources may be network elements and CPUs in the system. Data such as usage records are collected and analysis of capacity planning is performed based on present characteristics. Security functions are provided wherein the network is protected against unauthorized use. Security mechanisms built in to the network management include enhanced fraud security coding encryption and user passwords. Configuration management, i.e., how resources are allocated, is another function of the NOC. Fault detection and management are provided for by the NOC. Problems are isolated and reported to operations personnel who can react to the problems.
In another embodiment of the invention, a method of performing a call setup procedure in a mobile satellite system from a call initiated by a mobile communication system (MCS) to a destination served by a public switched telephone network, includes the steps of initiating the call by the MCS, the MCS formatting and transmitting an access request message via a random access channel, and receiving by the central controller the access request message, and transmitting frequency assignments to the MCS and to the SCSO. The method also includes receiving by the MCS the frequency assignment, transmitting a scrambling vector message to the SCSO, and verifying by the SCSO the identity of the MET responsive to the scrambling vector. Upon successful verification, the method includes the steps of switching by the SCSO and the MCS from call setup mode to voice mode, transmitting by the SCSO voices frames to the MCS including a voice activation disable signal to disable a voice activation timer in the MCS for at least 3 super frames, and transmitting a destination number to the PSTN. The method also includes the steps of transmitting by the SCSO an enable signal to the MCS to re-enable the call activation timer in the MCS, and establishing voice communication between the PSTN and the MCS.
In another embodiment of the invention, a method of performing a call setup procedure in a mobile satellite system from a call initiated by a destination served by a public switched telephone network (PSTN) to a mobile communication system (MCS). The method includes the steps of receiving by the SCSO a call from the destination served by the PSTN, transmitting by the SCSO to the central controller a channel request using interstation signaling, determining by the central controller an identity of the MCS responsive to the destination number, and transmitting a call announcement via a random access channel. The method also includes the steps of acknowledging by the MCS the call announcement via the random access channel to the central controller, transmitting frequency assignments to the MCS via the random access channel and to the SCSO via an interstation signaling channel, and transmitting an access security check field used to verify the MCS""s identity. The method also includes the steps of receiving by the MCS the frequency assignment, and transmitting a scrambling vector message to the SCSO, verifying by the SCSO the identity of the MET responsive to the scrambling vector, and upon successful verification, transmitting by the SCSO a ring command to the MCS. The method also includes the steps of receiving by the MCS of the ring command, generating a ringing signal to a MET user, and transmitting a ring command acknowledgement to the SCSO. The method also includes the steps of receiving by the SCSO the ring command acknowledgement from the MCS, and once the call setup is complete, transmitting by the MCS voice frames to the SCSO including a voice activation disable signal to disable a voice activation timer in the MCS for at least super frames. The method further includes the steps of upon detection of the MCS switching to a voice frame mode, switching by the SCSO to the voice mode, and transmitting a voice activation enable signal to the MCS to re-enable the call activation timer in the MCS, and establishing voice communication between the PSTN and the MCS.
The present invention also solves the following objectives:
To determine how a system designed to the standards will deal with internal failures that might jeopardize satisfactory operation.
To determine the criteria and methods used to prevent failures.
To determine how an AMS(R)S system might accommodate traffic fluctuations while meeting performance specifications.
To help in establishing the probability of, and performance criteria for, external resource provisioning to AMS(R)S from non-AMS(R)S systems.
To aid in determining the susceptibility of AMS(R)S to interference and how it recovers when it does happen.
To aid in establishing specifications for non-AMS(R)S systems that will ensure that a suitable preemptive capability for AMS(R)S is established and minimize the likelihood of interference.
In accordance with this embodiment of the invention, a priority and preemption method/system performs a priority and preemption process, and includes the steps of satisfying a resource acquisition request from a reserve pool for an external system, and when the resource acquisition request cannot be satisfied from the reserve pool, requesting additional unused frequencies, and when the additional unused frequencies are not available, requesting to preempt active calls. The method also includes the step of replenishing the power and the frequencies received from the frequency controller, the data hub and/or the independent operations controller when the frequencies are no longer needed by the priority and preemption system.
These, together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully herein described and claimed, with reference being had to the accompanying drawings forming a part hereof wherein like numerals refer to like elements throughout.